This project investigates the mechanisms of initial resistance to infection and generation of protective immunity to bacteria. The immune response to Francisella tularensis strain LVS, a gram negative facultative intracellular bacterium, is being characterized in terms of the cell types involved and their products (cytokines and antibodies). Results to date have demonstrated that mice which survive sublethal subcutaneous (sc) or intradermal (id) infection are solidly immune to subsequent lethal intraperitoneal (ip) or intravenous (iv) infection. Initial resistance to id infection is dependent on the production of Interleukin 12, tumor necrosis factor and interferon gamma by macrophages and natural killer cells, respectively, since scid mice that lack mature T or B lymphocytes are able to survive sc or id infection for about 20 days. Studies in athymic nu/nu mice further demonstrate that mature T cells are required for long term resolution of id infection and survival. Experiments using either mice depleted of CD4+, CD8+, or T cells, or scid mice reconstituted with various T cell subpopulations, demonstrate that either CD4+, or CD8+ T cells are sufficient for generation and expression of specific protective immunity to LVS. Cytokine production by T cells appears to be a major protective mechanism of protection has also been characterized in murine responses to Francisella: very strong specific protective immunity to LVS develops in both normal and nu/nu mice, but not scid mice, within 2-3 days after id priming with LVS. These results implicate a cell type present in nu/nu mice but not in scid mice that are able to generate immunity rapidly. In fact, both in vivo depletion studies and transfer studies suggest a major role for B220+ cells that is independent of antibody production. The mechanism(s) of this rapid generation of immunity may have important implications for development of vaccines to be used during community outbreaks of bacterial diseases. In addition, novel vaccine strategies are being studied in this model. We have demonstrated that intramuscular inoculation of mice with chromosomal LVS DNA results in the generation of strong protective immunity to a lethal LVS challenge, but not to an irrelevant bacterial challenge. The nature and mechanism of this "naked DNA" induced protection is under further study.